The present invention relates to an impact printing apparatus which employs a set of interchangeable type elements to print out a desired kind of alphanumeric characters and the like in conformity to the kind of expected input data. More particularly, the present invention is concerned with an impact printer which can identify each specific type element mounted thereon to optimumly match various printing conditions to the identified type element.
A serial impact printer is known of the type which is furnished with a set of interchangeable type elements to be operable with any one of the type elements which matches with a kind of expected input data as regards the alphanumeric characters and the like carried thereon. In this type of serial printer, the printing pressure and pitch should preferably be controlled in an optimum manner for the size and style of types and the kind of other symbols which are carried on each type element. Also, the serial printer has to perform a type selection control for selecting types on a type element in response to input data in accordance with type codes prescribed by ASCII or JIS. Thus, the printer stores control programs each corresponding to a specific type element while each type element is provided with a coded mark for discrimination. When loaded in the printer, a type wheel is distinguished from the others through its own coded mark and a control program matching with the kind of the specific type element is selected.
A prior art impact printer having such an additional function includes a type element in the form of a wheel which is provided with a mark constituted by a magnetic piece (or a plurality of coded marks) in a suitable location thereof, and a magnetic sensor for magnetically sensing the mark or marks. These component parts are mounted on a carriage together with a type selecting motor, the type element being detachably mounted on the output shaft of the motor. The carriage is movable horizontally along a platen which is in turn movable to feed a sheet of paper vertically in a stepwise manner. While the type element is rotated from its home position or reference position for a type selection control, the magnetic sensor detects the mark so that the kind of the type element is automatically discriminated based on the angular distance from the home position to the mark (or reading the coded data).
In this prior art serial printer, however, the output of the sensor must be fed from the moving carriage over to a stationary part of the printer by a special flexible cable. The carriage carries thereon an electromagnet for driving a hammer and other drive lines as well as the motor for driving the type element, all of which are naturally wired to a power supply individually. Therefore, wiring the moving carriage to the stationary part of the printer increases the total number of wirings, which should preferably be as small as possible from the structural viewpoint. For this reason, the sensor signal line is frequently laid adjacent to the various power source lines. The output voltage of the magnetic sensor is as low as about several to several tens of millivolts and, therefore, it is fed to a control system in the stationary part of the printer via a carriage cable after being converted into a several volt, several milliampere signal by an amplifier. With such a jammed cable arrangement, the sensor output tends to involve inductive noise affected by the pulsing drive signals of several amperes which flow through the adjacent power source lines, resulting an error in the identification of the type wheel.
The carriage repeats sharp acceleration, stop and return throughout its printing operation. Where the sensor is mounted on such a carriage, a specially designed mounting means is indispensable to ensure a sufficient accuracy in relative position between the sensor and the mark on the type element. The amplifier for amplifying the sensor output and a power supply for the exclusive use of the amplifier must be mounted on the carriage which undergoes considerable vibration. Furthermore, the increase in the number of elements mounted on the carriage is contradictory to a general demand for a compact construction and, therefore, quite disadvantageous in the aspect of control, assembly and maintenance.